In a projection x-ray imaging system, the object to be imaged is placed between an x-ray source that is typically a point source and an array of x-ray detectors. When such arrays are densely packed, the individual detectors are referred to as pixels. In general, the point illumination source generates an x-ray field that is roughly isotropic. That is, the number of x-rays that are generated per unit solid angle at the source is roughly constant. The array of detectors is typically flat, and hence, the solid angle subtended by each detector varies across the array, since the various detectors have different distances and orientations relative to the source. For example, consider a system in which the source is located over the center of the detector array and oriented such that the center detector in the array is oriented at right angles with respect to the ray from the center of the detector to the source. The solid angle subtended by the detectors varies with the detector's distance from the center of the array. The detectors that are nearer the edge of the array, and hence, far from the center, receive a lower x-ray intensity than those nearer the center.
Ideally, the projection image formed by the array when an object is placed between the x-ray source and the array reflects the variations in x-ray absorption or scattering by the object without illumination artifacts. However, the above-described illumination variation in x-ray intensity generates artifacts, since an object that has a uniform absorbency across the object would appear to have a density that varies with position if the image is not corrected for the intensity variation.
In addition to the illumination variation discussed above, the image must be corrected for variations in the gain and offset of the various x-ray detectors in the array. The image sensors used in these systems are generally imperfect. Both the dark current and gain varies from pixel to pixel in the array. This variation is often called the “fixed pattern noise” or FPN. This variation is typically corrected by taking dark field (illumination off) and bright field (illumination on) images and creating a gain and offset value for each pixel. A table of these parameters is stored in a memory. These values are used to correct each pixel value prior to sending that pixel to the image user. It should be noted that this correction process also automatically calibrates the illumination variation discussed above as well.
If the relative position of the x-ray source and the detector array is fixed, such a lookup table correction procedure provides a viable method for correcting the image. However, in systems in which a large number of images are taken with different orientations between the x-ray source and the detector array, this procedure becomes impractical because a table of correction values must be stored for each possible orientation.